How can I put a custom property into my pie menu?
It appears you are right clicking view source in the custom property in the UI.  Copy data_path will give you the path,  eg. ["prop"] where the property name is prop.  In the material properties panel the rna_item will be the active material, in the object, the context.object, in the data context.object.data, in the scene context.scene etc.

The code you are seeing is setup to handle the different object types that can have a custom property.  escape_identifier most likely handles garbage property names.

The pie menu template.  A custom property "prop" has been added to the scene.  The 'rna_item' equates to context.scene, the key to "prop":

class VIEW3D_PIE_template(Menu):
    # label is displayed at the center of the pie menu.
    bl_label = "Select Mode"

    def draw(self, context):
        layout = self.layout

        pie = layout.menu_pie()
        pie.prop(context.scene, '["prop"]')
        # operator_enum will just spread all available options
        # for the type enum of the operator on the pie
        pie.operator_enum("mesh.select_mode", "type")